Voltage Crowbar

ABSTRACT

The present invention provides for an electronic barrier device, which can form part of a further device such as an isolating barrier or a zener barrier, and comprising a voltage limiter such as at least one zener device for voltage limitation in a circuit during a fault condition, the barrier device including a crowbar device arranged to latch across the at least one zener device to reduce power dissipation in the at least one zener device in the circuit fault condition, wherein the crowbar device is arranged to latch responsive to a change in a current sensed in the barrier device.

The present invention relates to a voltage shunt device such as alsoknown as a voltage crowbar and, in particular, but not exclusively, to avoltage shunt device for use in power-dissipation reduction in a barrierdevice for intrinsically safe systems and such as found in isolatingbarriers or zener barriers.

Barrier devices commonly comprise voltage limiters, such as a zenergroup providing voltage-limitation, a fuse serving to limit the currentin the Zener, and limit resistors on an output leg of the device servingto limit the output current arising at the defined zener clampingvoltage. Zeners, and resistors must be used at no more than ⅔^(rd) oftheir maximum rating, and the fuse must be considered to remain intactwith a sustained current at 1.7 times its rating.

However, such an arrangement can prove problematic since the powerconsumption in the zeners in fault condition, i.e. when the zeners areclamping the voltage, can become considerable and the heat generated canprove difficult to dissipate, particularly in a small enclosure typicalof an isolating barrier or a zener barrier device.

For example, and noting the above rating characteristics, for asustained current of 70 mA, a 100 mA fuse would be required. If theoutput voltage is to be 20V max, 20V zeners would be necessary. Thisarrangement will generate (0.1 A×1.7)×20V=3.4 W of heat to dissipate infault condition and so will require a rating of 5.1 W continuous for thezener. Dissipating such power and providing a suitable zener might beimpossible. To alleviate such a problem of power dissipation, a knownsolution is to use a so-called voltage crowbar device. Such devicesoperate to detect the voltage at the zener before it reaches the zenerminimum voltage (according to the zener tolerances) and if that voltageexceeds the limit, a latching short circuit is fired across the zener.As an example, such a short circuit could provide 1V across the crowbarand with a current of 100 mA×1.7, the dissipated power in the crowbardevice will then only be 0.17 W—as compared with the 3.4 W that hadpreviously arisen in the Zener(s).

For further illustration of the current art, and turning to FIG. 1,there is provided a diagram of a basic barrier device in which anundefined voltage with undefined current capability is present at aninput 104 such that some, or all, of its power is available on an output105. The voltage is subjected to zener clamping 101 at the Zener voltageVz, to guarantee a maximum value of ≤Vz. This now protected voltage,while passing current through a limit resistor 103 restricts the maximumcurrent available on the output 105 to ≤Vz÷RI.

As noted above, such values should remain valid at the followingconditions: all components must be ⅔ rated in their power parameters.Therefore to protect the zeners from this rule being breached, a seriesfuse 102 serves to limit the continuous current to 1.7 times the nominalfuse value (that factor is deemed to be a safe one and is used as norm).For practical reasons the fuse rating must be at least the same as theoutput short circuit current, and preferably exceed it by a safetyfactor, for long term reliability.

This means that, if a zener sees a continuous power of maximum ⅔ of themaximal allowed continuous power as specified in the datasheet, it canonly fail in reduced value (short circuit included), never in increasedvalue (open circuit included).

Also, if a resistor sees a continuous power of maximum ⅔ of the maximalallowed continuous power as specified in the datasheet, it can only failin increased value (open circuit included), never in reduced value(short circuit included).

Further, both component types must also be able to withstand 1.5 timesthe possible transients as per their datasheet.

Still further, temperature de-rating will have to be applied, andsegregation distances will be a safety requirement.

Finally, the distance in the barrier, from an input boundary 106 to anoutput boundary 107, could be part of a zener barrier or part of a morecomplex apparatus, an isolating barrier for example, and such that anintrinsically safe signal can be available after the output boundary.

If choice of zener was limited to practical zener device for theapplications, and since, most power zeners are either limited to 3 Wcontinuous for surface mounted types, or 5 W continuous for the leadedcomponents, the maximum allowed continuous power dissipated would thenbe limited respectively to 2 W and 3.3 W.

However, in a fault condition, the continuous power requiringdissipation can be well over these values.

Even if the limiting elements could withstand such power, the enclosuremay not be able to dissipate the generated heat.

A known solution to this is to let the zeners absorb transients but whenthe continuous power is experienced above their rating, a shunt elementtriggers and shorts the zeners, in turn blowing the safety fuse. Thisthen has the effect of annihilating unsafe power.

The shunt element is of the latching type and must remain on until thepower is removed (mostly when the fuse is blown).

Additionally the shunt must be triplicated and infallibly constructedsince for ia and ib circuits, two countable faults could still occur.

In fault conditions, when the voltage's value approaches the zener'sminimum clamping voltage, if a shunt or short circuit is applied acrossthe zeners, the only power dissipated before the fuse is blown is theinput current in this situation times the shunt voltage drop, resultingin lower power that would be otherwise dissipated in the zeners.

Further details of such a known crowbar arrangement are illustrated inFIG. 2, where, again an undefined voltage with undefined currentcapability is present at an input 204 and some or all of its power isavailable on an output 205. The voltage is again subjected to zenerclamping 201 to guarantee a maximum value≤Vz. This now protectedvoltage, while passing current through a limit resistor 203 restrictsthe maximum current available on the output 205 to ≤Vz÷RI.

However, and according to the crowbar functionality, the input voltageis compared to a reference voltage 230 by a comparing element 231.

If the input voltage is higher than a predetermined safe value, thecomparing element will close the shunt element 220. This shunt elementis of the latching type and will remain closed until its current isremoved.

The maximum power is now only (1.7×the fuse rated current)×(closed shuntvoltage).

Reference is now made to FIG. 3 which illustrates the operating rangesassociated with such a shunt.

The zeners have a nominal voltage tolerance 301 302. Additionally thenominal zener value decreases with the temperature. This gives Vzh, themaximum zener voltage 311 and Vzl, the minimum zener voltage 312. Vzh isthe safety voltage 305 used to describe the intrinsically safe output205.

The incoming voltage will be subject to noise 341. Practically, withoutthe shunt, the noise range must be situated underneath the zeners toprevent them conducting repetitively. The useful voltage will be Vzu,the minimum noise level 342.

The shunt circuitry has also an operating tolerance range 321. Typicallysuch a shunt system will have a positive temperature coefficient. Thisgives Vsh, the maximum shunt trigger voltage 331 and Vsl, the minimumshunt trigger voltage 332.

As the shunt must operate before any zener takes over, Vsh must be lowerthan Vzu with a small safety margin.

The shunt is a latching circuit and should not be triggered on anyincoming noise. The incoming voltage will be subject to noise 351.Practically, the noise range must be situated well underneath the shuntminimum trigger point Vsl to prevent it to blow the fuse. The usefulvoltage will be Vsu, the minimum noise level 352.

Examples of element values are:

Fuse 202: 1 A

Zener value 201: 10V total

Zener tolerance 301, 302: ±5% (full temperature)

Limit resistor 203: 20Ω

Voltage noise levels 341, 351: 1V

Shunt trigger point tolerance 321: 0.4V

Shunt on voltage drop: 1V

Vzh=10V+5%=10.5V Vzl=10V−5%=9.5V

With a safety margin of 0.5V: Vzu=9.5V−0.5V−1V=8.0V

With a safety margin of 0.5V: Vsh=8.0V−0.5V=7.5V

Vsl=7.5V−0.4V=7.1V

With a safety margin of 0.5V: Vsu=7.1V−1V=6.1V

The safety voltage max 305 is Vzh=10.5V

The useful voltage max 325 is Vsu=6.1V (58% of Vzh)

Maximum zener power (no shunt): Pz=(10V+5%)×(1 A×1.7)=17.8 W

Maximum shunt-on power: Ps=1V×(1 A×1.7)=1.7 W

As can therefore be seen with reference to the above exemplary values, agreat part of the maximum voltage available cannot be used as it issupressed to ensure a smooth operation of the product. There is a greatdistance between the maximum declared safe voltage and the practicalvoltage available.

Additionally, this proves particularly disadvantageous since, with thecircuit being triplicated, the use of an expensive reference can lead toa threefold increase in cost.

If a fault develops, which takes any substantial current, the voltageafter the fuse is likely to drop. This means that the trigger circuit isless likely to activate. In consequence every shunt element must be ableto take the full power available with a 1.7 times the nominal fusecurrent. While available zeners might not be dimensioned for that, ashunt element could be.

Also, if in one chain of zeners, one fails short or low value, theremaining zeners may clamp at a lower voltage that the shunt triggervoltage.

Thus, the operation of known crowbar devices can prove problematic andlimited since, as the output voltage is desired to be as high aspossible, and the clamping voltage as low as possible, specificallyselected zeners are required which can increase costs threefold. Also,accurate and stable references must be employed within the voltagecrowbar detection circuits, ideally in duplicate or triplicate, whichhas further cost implications. Therefore such known voltage-basedcrowbars are costly and can prove overly sensitive as they areconfigured just near the tolerances limits

The present invention therefore seeks to provide for a voltage crowbardevice having advantages over known such voltage crowbar devices.

According to one aspect of the present invention, there is provided anelectronic barrier comprising a voltage limiter, for example employingzener diodes for voltage limitation in a circuit during a faultcondition, the barrier device including a crowbar device arranged tolatch across the voltage limiter to reduce power dissipation in thevoltage limiter in the circuit fault condition, wherein the crowbardevice is arranged to latch responsive to a current sensed in thebarrier device.

The invention is advantageous in that by sensing the current in theline, instead of sensing the voltage, the operation of the crowbar isindependent of voltage limiter tolerances. Also, current sensingbenefits from a wide range of current available within the circuit,which is well beyond the required current, as so the tolerances arisingin current sensing are inherently greater than with voltage sensing

As an example, the voltage limiter can comprise at least one zenerdevice.

On this basis, the crowbar functionality of the present invention can beimplemented with normal tolerance components and can further exhibitreduced sensitivity. Sensitivity can also further be improved since,while a voltage spike could occur easily before reaching the zenerover-clamping voltage, a current spike cannot occur because the maximumvoltage is Zener-defined and therefore the maximum current through forexample the serial resistors is limited by that voltage.

Preferably, the barrier device includes a fuse and/or limiting resistor.In such an arrangement, the sensed current can comprise that passingthrough the fuse.

Also, the device can advantageously comprise an intrinsic safetybarrier.

Yet further, the crowbar device is arranged, when latched across thezener device, to remove all power dissipation within the zener device.In particular, the current is diverted through the crowbar, and with aresidual voltage of around 1V or less across the crowbar, there can begreatly reduced power dissipation.

According to another aspect of the present invention is provided acrowbar device arranged to latch across a voltage limiter, such as atleast one zener, of a barrier device, to reduce power dissipation in thevoltage limiter, wherein the crowbar device is arranged to latchresponsive to a sensed current in the barrier device.

According to yet another aspect of the present invention, there isprovided a method of reducing power dissipation in a voltage limiter,such as at least one zener, of a barrier device, including the step oflatching a crowbar device across the voltage limiter to reduce powerdissipation in the voltage limiter, and wherein the step of latching thecrowbar device occurs responsive to the sensing of a current in thebarrier device.

The invention is described further hereinafter, by way of example only,with reference to the accompanying drawings in which:

FIG. 4 the circuit diagram of a barrier device employing a voltagecrowbar device according to an embodiment of the present mention; and

FIG. 5 is a voltage diagram illustrating the operational tolerances of avoltage crowbar device such as that illustrated in FIG. 4.

As will therefore be appreciated, the invention involves triggering theshunt on a sensed current rather than an input voltage.

Turning to FIG. 4, which for ease of comparison has a similarconfiguration as the prior art arrangement of FIG. 2, an undefinedvoltage with undefined current capability is present at an input 404 andsome or all of its power is available on an output 405. The voltage issubjected to voltage limiting which, in this illustrated example,comprises zener clamping 401 to guarantee a maximum value ≤Vz. This nowprotected voltage, while passing current through a limit resistor 403restricts the maximum current available on the output 405 to ≤Vz÷RI.

In addition, the input current is sensed through a resistor 440 andevaluated by a comparison element 441. If the input current is higherthan a safe value, the comparison element will close the shunt element420. This shunt element is advantageously of the latching type and willremain closed until its current is removed. Additionally, because of thecircuit, closing the shunt consequently increases the current throughthe sense element 440 and it is therefore self-latching.

The maximum power dissipated is now across the shunt and is now only(1.7×the fuse rated current)×(closed shunt voltage). The power has thusbeen reduced, not by sensing the voltage, but rather by sensing thecurrent. That is, the both power has been reduced and also the triggerfor activation of the crowbar is different.

Normally the user is not operating the output in short-circuit but insome useful mode, which will be well below the maximum current availablethrough the fuse. This means that the current clamp can advantageouslyoperate at a much lower value than the nominal fuse value ×1.7.

Examples of practical operating ranges of such a shunt are illustratedin FIG. 5.

That is, the zeners have a voltage tolerance 501, 502. Additionally thenominal zener value decreases with the temperature giving Vzh, themaximum zener voltage 511 and Vzl, the minimum zener voltage 512. Vzh isthe safety voltage 505 used to describe the intrinsically safe output405.

The incoming voltage will be subject to noise 541. Practically, thenoise range must be situated underneath the zeners to prevent themconducting repetitively. The useful voltage will be Vzu, the minimumnoise level 542.

Since the shunt circuitry is not operating in the voltage range, thismeans that the useful voltage Vsu is the minimum noise level 542.

Practically it is desired to operate the output up to In, the maximumoperating current 570. In current mode, the noise level is much lowerthan in voltage mode. Allowing for a small current noise 591, and thesmall tolerances 561 on the current trigger, this permit to place Isl,the minimum current trigger level 571 very near the noise. One can setIs, the nominal current trigger level 581 just after Ish, the maximumcurrent trigger level 572.

The following are examples of possible values for illustrative purposes,when possible using the same values as our previous example, forcomparison:

Fuse 402: 1 A

Zeners value 401: 10V total

Zener tolerance 501, 502: ±5% (full temperature)

Limit resistor 403: 20Ω

Voltage noise level 541: 1V

Current noise level 541: 0.1V

Shunt trigger point tolerance 521: ±2%

Vzh=10V+5%=10.5V Vzl=10V−5%=9.5V

With a safety margin of 0.5V: Vzu=Vsu=9.5V−0.5V−1V=8.0V

The safety voltage max 505 is Vzh=10.5V

The useful voltage max 542 is Vsu=8.0V (76% of Vzh)

Maximum zener power (no shunt): Pz=(10V+5%)×(1 A×1.7)=17.8 W

Maximum shunt-on power: Ps=1V×(1 A×1.7)=1.7 W

Maximum current available: In=10.5V÷20Ω=0.53 A

With a safety margin of 0.1 A and the current noise:

Isl=0.53 A+0.1 A+0.1 A=0.73 A Ish=0.73 A+4%=0.76 A

With a safety margin of 0.1 A: Is=0.76 A+0.1 A=0.86 A

We can see in this example that the useful voltage available hasincreased from

58% to 76%, a substantial gain.

Various advantages arise from the use of an over-current triggered shuntas compared with the present art. In particular, more voltage isavailable at the output of the barrier device. Also, there is lessdistance between the maximum declared safe voltage and the practicalvoltage available

Operating with reference to an input current value, and because themaximum current is defined by a short on the limit resistor, it is easyto place the trigger level just outside that input current. As thecircuit must be triplicated, and there is no need for a reference, theoverall cost is not severely impacted by such requirement fortriplication.

Also, if a fault develops, which takes any substantial current, thecurrent after the fuse is likely to increase. This means that thetrigger circuit will activate. In consequence every shunt element mustnot be required to take the full power available with a 1.7 times thenominal fuse current. If, in one chain of zeners, one fails with ashort, or low value, the current will increase to the shunt triggervalue and therefore will automatically be safe.

By setting the shunt trigger current very near the practical maximumoutput current, it will be very near the fuse rating or lower withoutbeing affected by the factor 1.7.

Various potential fail-scenarios are outlined below, with discussion ofthe respective reactions of a voltage sensed shunt as in the currentart, and a current sensed shunt according to an embodiment of theinvention.

First, it is envisaged that one zener might fails, either with a lowervalue or short. Then, for a voltage sensed shunt, the shunt may nevertrigger as the voltage is likely to decrease, and also any otherzener(s) will see the full power available. However, with a currentsensed shunt, it remains fail-safe since the shunt will trigger early asthe current will rise.

Next it is envisaged that one shunt element fails, it can fail in anymode. If it fails open, a voltage sensed shunt will remain fail safesince the shunt is triplicated. In a similar way, for a current sensedshunt, fail safe operation will be retained as again the shunt istriplicated

If one shunt element fails, it can fail in any mode. If it failsresistive or short then for a voltage sensed shunt, the element may haveto take all of the available power. The triplication will not help asthe voltage is likely to decrease, therefore no trigger action isguaranteed. However, for a current sensed shunt, it will remain failsafe and, as the element will take more current, the rest of thetriplicated shunt can or will trigger.

Further, if one comparison element fails, it can fail in any mode andwith a voltage sensed shunt, it will be in fail safe mode as the shuntis triplicated, or else it will be triggered. For a current sensedshunt, again, it will remain in fail safe mode as the shunt istriplicated, or it will be triggered.

If one reference fails, it can fail in any mode and, as above, thevoltage shunt will be in fail safe mode and, as the shunt is triplicatedor it will be triggered. As there is no reference, in such a fail mode acurrent sensed shunt is not applicable.

As a final example, if the sense resistor fails, it can fail with ahigher value or open and in such case a voltage sensed shunt will not beapplicable. However, a current sensed shunt will be in fail safe and theshunt will trigger early as the current will be sensed at a higherlevel.

It should of course be appreciated that the invention is not restrictedto the details of the foregoing embodiments. For example, the inventionneed not be embodied solely in a zener barrier device per se, but canalso related to associated apparatus using a barrier arrangement as apossibly integral part of the circuitry, such as for example isolatingbarrier devices.

Further, the crowbar could additionally be arranged to reduce power inother components of the barrier, such as for example the limit resistor.

1. An electronic barrier device comprising: a voltage limiter device forvoltage limitation in a circuit during a fault condition, and a crowbardevice arranged to latch across the voltage limiter device to reducepower dissipation in the voltage limiter device in the circuit faultcondition, wherein the crowbar device is arranged to latch responsive toa change in a current sensed in the barrier device.
 2. The electronicbarrier device of claim 1, further including a fuse and wherein thesensed current comprises a current through the fuse.
 3. The electronicbarrier device of claim 1, further including an output line and alimiting resistor in the output line.
 4. The electronic barrier deviceof claim 1, further comprising an intrinsic safety barrier.
 5. Theelectronic barrier device of claim 1, wherein the crowbar device isarranged, when latched across the voltage limiter device, to remove allbut negligible power dissipation within the voltage limiter device. 6.The electronic barrier device of claim 5, wherein the crowbar device isarranged such that almost all of the power dissipation during thecircuit fault condition occurs across the crowbar device.
 7. Theelectronic barrier device of claim 1, wherein the voltage limiter devicecomprises at least one zener device.
 8. The electronic barrier device ofclaim 1, wherein the crowbar device is arranged to reduce powerdissipation in at least one non-voltage limiter device within thebarrier device.
 9. The electronic barrier device of claim 8, wherein oneof the at least one non-voltage limiter device the crowbar device isarranged to reduce power dissipation in is a limit resistor. 10.(canceled)
 11. (canceled)
 12. An electronic barrier device comprising: acrowbar device arranged to latch across a voltage limiter device of thebarrier device, to reduce power dissipation in the voltage limiterdevice, wherein the crowbar device is arranged to latch responsive to achange in a current sensed in the barrier device.
 13. The electronicbarrier device of claim 12, wherein the crowbar device is arranged, whenlatched across the voltage limiter device, to stop power dissipation inthe voltage limiter device.
 14. (canceled)
 15. The electronic barrierdevice of claim 12, wherein the crowbar device is arranged to reducepower dissipation in at least one non-voltage limiter device within thebarrier device.
 16. The electronic barrier device of claim 15, whereinone of the at least one non-voltage limiter device the crowbar device isarranged to reduce power dissipation in is a limit resistor. 17.(canceled)
 18. (canceled)
 19. A method of reducing power dissipation ina voltage-limiter of a barrier device during a circuit fault condition,including the step of: sensing a change in current in the barrierdevice, and latching a crowbar device across the voltage limiter toreduce the power dissipation in response to the sensing of the change incurrent.
 20. The method of claim 19, wherein the step of sensing thechange in current includes sensing a change in current through a fusedevice of the barrier device.
 21. The method of claim 19, wherein thecrowbar device, when latched across the voltage limiter device, removesall but negligible power dissipation within the voltage limiter device.22. The method of claim 21, wherein almost all of the power dissipationin the barrier device during the circuit fault condition occurs acrossthe crowbar device.
 23. The method of claim 19, wherein the voltagelimiter device comprises at least one zener device.
 24. The method ofclaim 19, wherein latching the crowbar device across the voltage limiterto reduce the power dissipation in the voltage limiter simultaneouslyreduces the power dissipation in at least one non-voltage limiter devicewithin the barrier device.
 25. The method of claim 12, wherein one ofthe at least one non-voltage limiter device that the crowbar devicereduces power dissipation in is a limit resistor.